ipatests: Update ipa-adtrust-install test
update after change in implementation of `krb_utils.get_principal()` now using GSSAPI
Related: https://pagure.io/freeipa/issue/9575
Signed-off-by: Erik Belko <ebelko@redhat.com>
Reviewed-By: Michal Polovka <mpolovka@redhat.com>